Bamidbar:15:23 – All that the Lord commanded you through Moses, from the day on which the Lord commanded and from then on, for all generations.
Gemora Kiddushin 29a: The School of R. Ishmael taught: whenever ‘command’ (צו) is stated its only purpose is to denote exhortation for that time and for all time. [Just for that time is shown by the verse in Parshat VeEtchanan:] “But command (צו) Joshua, and encourage him, and strengthen him.” Then and for all time [is shown by the verse in Bamidbar 15:23] ” as it is written, from the day on which the Lord commanded and from then on, for all generations.”
Torah Temimah Colloquial Translation on Note #61:
The Gemora is not trying to say that the verb ‘to command’ (צו) implies that the command is an eternal command [as opposed to a command just for that time and place.] Behold, we see places where the verb “command” is used for just that time and place as in the verse “he commanded Yosef regarding his household” [Bereishis-44:1] or the verse “and Pharaoh commanded all his people” [Shmos-1:22] and other many similar verses.
Rather, the Gemora is saying that when the verb “command” is used regarding laws and ordinances from God, then the verb “to command” is using to show that the commandment is eternal. The reason why the verb ‘command’ is different when used by God versus when used by a man is simple. A human being doesn’t have the power to command something forever; because he, himself, is not eternal. Additionally, over time the circumstances regarding whatever the particular person commanded will change and there will no longer be a need for that commandment. Or, on the other hand the circumstances would change and there would be a need to change the commandment. This, however, is not the case with the Holy One, Blessed be He. He exists forever and ever. Also, every single thing that emanated from His mouth [so to speak] is something that has eternal existence and strength.
It does merit further investigation, however, why the Gemora mentioned the verses that it does mention to prove its point. First it mentions the word “command” regarding Joshua that he should be encouraged and strengthened. Then the Gemora mentions our verse where it uses the word “command” and says explicitly “…for all generations”. [This is not a good proof because] it implies that if our verse had not added the words “…for all generations”, then the word “command” would not have been sufficient for me to know that the commandment was for all generations. If this is the case, how can this verse be used as a proof for the statement that the word ‘command’ implies for all generations?
The explanation is that one time [is sufficient] for the Torah to reveal the fact that the word “command” when used regarding laws and ordinances [signifies forever] and it is leveraged to teach the use of the word in all other places. This teaching in our Gemora is using the “gezerat sheva” methodology as follows: “just as it says here the word ‘command’ and it means for generations, so similarly when it uses the word ‘command’ in any other place, it also means for all generations.
Editor’s Note: The Torah Temimah insightfully points out that the verse that the Gemora uses actually seems to prove the exact opposite of the Gemora’s point. It seems to me that the Torah Temimah thinks critically about every statement in the Gemora and accepts nothing at face value without analyzing it. In this case, he shows that the Gemora’s logic is comparable to a gezerah shavah and it is valid when viewed from that perspective.
Here is an excerpt from the Wikipedia article on gezerah shavah: The gezerah shavah (“Similar laws, similar verdicts”) is the second rule of Hillel and of Rabbi Ishmael, and the seventh of Eliezer ben Jose HaGelili. This may be described as argument by analogy, which infers from the similarity of two cases that the legal decision given for the one holds good for the other also. The term “gezerah shavah” originally included arguments based on analogies either in word or in fact. Before long, however, the latter class was designated as “hekkesh,” while the phrase “gezerah shavah” was limited to analogy in the case of two different Biblical laws containing a word common to both. The gezerah shavah was originally restricted to a δὶς λεγόμενον, i.e., a word occurring only in the two passages offering the analogy. Since such a word is found nowhere else, there is no reason to assume that it bears different meanings in the two passages. The gezerah shavah consequently attaches to the word in the one passage the entire sequence of ideas which it bears in the other.